Sustained quasi-steady turbidity current : outcrop evidence from the Pliocene peri-Adriatic foredeep (Cellino Fm., Central Italy)

The aim of this work is to investigate the nature of the numerous, very thick deep-water sheet-sandstones that dominate the lower portion of the Cellino Formation (Central Italy). The studied turbidite system (about 2,500 m thick) represents the Lower Pliocene turbiditic filling of the outer Abruzzo sector of the Periadriatic foredeep. The foredeep was affected by compressional deformation linked to the overall migration of the chain-foredeep system toward the east. Tectonic activity was mostly coeval with the sedimentation and propagated towards the foreland; thrusting became progressively younger from W to E. The Cellino Basin has been intensely explored, being the site of hydrocarbon-bearing sands. The turbidite beds can be distinctively resolved in the well logs and correlated to the measured sedimentary sections on outcrop. Based on well log correlation, tens of individual beds up to 23 m thick have been traced along the axis of the basin over distance in excess of 100 km and, perpendicularly to the basin, over distance of 30-40 km (Carruba et al. in press), with sand volumes on the order of a few 10’s Km3 (10 – 80 Km3). Palaeocurrent data taken from basal flute structures indicates southerly-directed flows, parallel to the depocentral axis of the basin. The thickest beds show a basin-wide extension, onlapping the basin margins without significant thickness variation. The internal organization of the studied megabeds provides evidence for occurrence of long-lived flows and suggests deposition by gradual aggradation from sustained currents (sustained turbidity current; Kneller and Branney, 1995). The following features have been argued to be characteristic for sustained currents: (i) turbidite beds of extraordinary volume and thickness, (ii) very thick massive basal division (0.5 – 6 m thick), (iii) very frequent alternation of structureless and laminated intervals associated to internal scour surfaces, (iv) thick massive mudstone cap (1-10 m) that terminates the vertical organization of the sedimentary structures, (v) crudely developed grain-size profile that is overall upward fining (normally graded), (vi) abundant organic matter, (vii) extensive water-escape features. The very thick massive basal division observed in the studied megabeds can be explained with progressive aggradation and absence of traction at the depositional flow boundary. The very frequent alternation of structureless and laminated intervals observed within the studied deposits, and their internal scour surfaces reflect temporal variation in flow velocity and sediment flux within the same current, as indicated by the discontinuity of the scour surface and the constant grain size above and below the surfaces. The graded upper part of the studied megabeds (a thick massive mud cap terminates the vertical organization of the sedimentary structures) represents the deposits of the waning stage of the current. Assuming a quasi-steady flow scenario we can explain the nature of the numerous very thick megabeds within the Cellino Fm considering that the determining factors of the thickness of the studied deposits are the confinement of the basin and the rate and the duration of deposition, which may proceed as long as the current maintains a flux of grains towards the site of deposition. The origin of these large-volume turbidity currents and their high rate of occurrence can be related to an interaction of many factors and external controls, which are typical of the ancient foredeep basins (Mutti et al., 2003). Our data suggest that the studied megabeds could be originated from catastrophic floods and sediment failures during relative falling- and low-stand stage of sealevel forced by dramatic tectonic uplift of basin margins. Where the mountains fronts are close to shoreline, floods would be able to carry the majority of sediment load directly to the sea; the final depositional area of the ancient fluvial system that probably fed the Cellino basin lies in the deep water, far away from river mouths, and it is recorded by basinal turbidite sandstones and megabeds. Although an understanding of climatic controls is extremely difficult on the basis of available data, high-frequency climatic pulses (that provided the water through which sediments were periodically flushed to the Periadriatic foredeep by flood-related process) could explain the amount of stacked megabeds. In this model, lower-frequency tectonically-forced cycles of uplift/denudation account for sediment availability through time. Consequently, the lower portion of the Cellino Formation could correspond to a stage of a single uplift/denudation cycle. In this stage, the tectonically active Cellino basin reaches its highest instability because the elevation of drainage basins is maximum and its proximity to the shoreline minimum.